This invention relates to a method and an apparatus for driving sealing elements in a packing machine for the continuous manufacture of filled and sealed bags.
In packing machines which produce filled bags it is a desideratum to vary the size of the bags and thus the quantity of goods contained therein. British Patent No. 904,519 has resolved this object by providing two webs of a wrapper material which are introduced to a sealing station by shaping shoulders. The sealing station is formed by two sealing rollers which may be moved towards and away from one another and which have edge portions for forming opposite longitudinal seams and also have a transversal sealing shoe for forming transverse seams in the bags. When the rollers of the sealing roller pair are pressed to one another, bags of a predetermined length are formed. Control means are provided for moving the sealing rollers away from one another to thus vary the length of the bags. In wrapper films made of heat sealable material, however, the sealing rollers have to be maintained at relatively high temperatures in order to ensure that airtight seams are produced. Such an arrangement may lead to excessive heat effects which may cause, for example, caramelization in case sugar products are being wrapped.
In the system according to U.S. Pat. No. 4,291,520 there are provided, in contradistinction to the apparatus disclosed in the above-discussed British patent, two separate sealing stations for the longitudinal seams on the one hand and the transverse seams on the other hand. The drawing of the wrapper sleeve is effected by means of separate drive arrangements. The transverse sealing shoes are driven separately and travel with the bag along a length portion of its travelling path. In case the wrapper sleeve is drawn intermittently, the effect on the heat output of the longitudinal sealing arrangement is substantial because the film material is moved non-uniformly through the sealing station and thus differently heated zones are formed. In this apparatus, however, the bag lengths may be varied relatively simply by changing the delivery speed of the film. It is, however, a disadvantage of this arrangement that a synchronization of all three component units, that is, the control of the longitudinal sealing means, the web drawing device and the motion of the transverse sealing shoes has to be ensured in case the bag length is to be changed.